This invention pertains to the sealing of heat sealable thermoplastic package wrapping film and more particularly, to the jaws that perform the sealing operation and wrapping machines which may have a plurality of packages in a tube or envelope of film in which it is necessary to produce simultaneously two seals and a cut therebetween. For example, product is fed into the machines and is wrapped in a tube or shroud of film which is still connected to the supply rolls. As the product, which becomes enshrouded in film progresses through the wrapping machine, at some point transverse seal jaws seal the film at the trailing edge of one package and the leading edge of another and simultaneously produce a cut in the film between the two seals.
There are several types of systems in the prior art to produce the above results, all of which seal and cut the packaging film by forcing heated sealing and cutting elements against an elastomeric cushioned bed with the film to be sealed and cut entrapped therebetween. There are wires or bands or combinations thereof which, with an impulse of heat, make the seal and the cut. The major difficulty with this type of system is that precise control of temperature is extremely difficult. If this system is used on polyethylene, it tends to gum up with molten film. If used with polyvinyl chloride film, the film tends to carbonize and build up on the sealing wires. The gumming or build-up is insulative and it interferes with the sealing process. Any attempt to protect the sealing elements, such as with a covering of Teflon fiberglass cloth, reduces the rate of heat transfer to the film and slows down the machine.
Additionally, there are hot knife sealing systems where a hot knife blade comes down, makes the cut in the film and simultaneously seals the two edges of the film together. The hot knife has also yielded a combination by using sealing bars, one on either side of the knife blade to make a fin-type seal. The main difficulty with this system is that the distance the knife blade protrudes is very critical if the jaw, when closed against the elastomer mating bed, is to produce the proper sealing pressure and the proper cutting pressure simultaneously. Further, when using the temperature required to make a clean cut, some types of polyethylene film soften to a point where the molten film will gum up between the knife and the sealing surfaces requiring frequent maintenance and cleaning. A variation of the above described sealing knife combination has a sharp knife which is fixed and protrudes below the sealing bars and cuts the film by forcing it into a slot in the elastomer bed. It is difficult to obtain good cuts with this system unless the knife is very sharp. This precludes applying a Teflon release coating to the knife, and polyethylene film tends to stick to it rendering the system inoperable.
Machines have also been built using heated metallic seal jaws which close against heated metallic seal beds, as seen in Smith, U.S. Pat. No. 4,807,426. The seal is produced in paper and/or foil laminates by crimping the material between the jaws and the metallic beds. Such machines can only be used on thermoplastic shrink film with difficulty because they may destroy the film in the seal region.